1. Technical Field
This invention relates to the art of fluid lubricated metal wear interfaces or contacts, and more particularly to the use of anti-friction solid film lubricants for such interfaces modified to withstand high unit scraping or bearing loads at high temperatures while functioning with either full or partial fluid lubrication.
2. Discussion of the Prior Art
The utility of certain solid film lubricants for bearings has been known for some time. U.S. Pat. No. 1,654,509 (1927) discloses use of powder graphite trapped or covered by a metal binder (i.e., iron, aluminum, bronze, tin, lead, babbitt, or copper) to form a thick coating; all of the metal is heated to at least a thermoplastic condition by melting or arc spraying to bury the graphite. The coating offers limited friction reducing characteristics. Unfortunately (i) the graphite is not exposed except by significant wear of the metal, thus never realizing significantly lower friction; (ii) the metal is in a molten condition prior to trapping or burying the graphite causing thermal effects and distortions; and (iii) oxides of the metal serve as the primary lubricant. The prior art has also appreciated the advantage of thermally spraying (by oxy-fuel) aluminum bronze as a solid film lubricant onto cylinder bore surfaces of an engine as demonstrated in U.S. Pat. No. 5,080,056. The lubricating quality of such coating at high temperatures is not satisfactory because (i) it lacks compatibility with piston ring materials which usually comprise cast iron, molybdenum coated cast iron, or electroplated hard chromium; and (ii) thermal spraying of the material by oxy-fuel is not desirable because of very high heat input necessitating elaborate tooling to rapidly dissipate heat to avoid distortion of its coated part.
One of the coauthors of this invention has previously disclosed certain solid lubricants operable at high temperatures, but designed for interfacing with ceramics, not metals, and generally at low load applications in the absence of any liquids. One solid lubricant disclosed comprised graphite and boron nitride in a highly viscous thermoplastic polymer binder spread in a generous volume onto a seal support comprised of nickel and chromium alloy. The formulation was designed to provide a hard coating which softens at the surface under load while at or above the operating temperature and functioning only under dry operating conditions. Thermoplastic polymer based formulations are unsatisfactory in meeting the needs of a loaded engine component, such as a cylinder bore, because the unit loads are significantly higher (approaching 500 psi), and the surface temperatures are higher, causing scraping. Another solid lubricant disclosed was halide salts or MoS.sub.2 (but not as a combination) in a nickel, copper, or cobalt binder; the coating, without modifications, would not be effective in providing a stable and durable anti-friction coating for the walls of an internal combustion cylinder bore, because the formulations were designed to operate under dry conditions and against ceramics, primarily lithium aluminum silicate and magnesium aluminum silicate, and, thus, the right matrix was not used nor was the right combination of solid lubricants used. Particularly significant is the fact that the formulations were designed to produce a ceramic compatible oxide (e.g., copper oxide or nickel oxide) through partial oxidation of the metal in the formulation. These systems were designed to permit as much as 300-500 microns wear. In the cylinder bore application, only 5-10 micron wear is permitted.
It is an object of this invention to provide a plasma sprayable powder for coating a light metal (e.g., alloys of either aluminum, magnesium, or titanium with silicon, zinc, tin, or copper, etc.) cylinder bore surface of an internal combustion engine, the powder having a soft metal encapsulating certain selected solid lubricant particles therein (CaF.sub.2, MoS.sub.2, LiF), and, optionally, having soft metal encapsulating hard, wear resistant particles. The encapsulation promotes improved fusion to the light metal bore surface and promotes a lace-like network of fusion metal between particles.
Another object is to provide a coating composition that economically reduces friction for high temperature applications, particularly along a cylinder bore wall at temperatures above 700.degree. F. when oil lubrication fails or in the presence of oil flooding (while successfully resisting conventional or improved piston ring applied loads).
Another object of this invention is to provide a lower cost method of making coated cylinder walls by rapidly applying a coating by plasma spraying requiring less energy and at reduced or selected areas of the bore wall while achieving excellent adherence and precise deposition with a larger powder grain size, the method demanding less rough and machine finishing of the bore surface.
Still another object is to provide a coated aluminum alloy cylinder wall product for an engine that (i) assists in achieving reduced piston system friction and reduced piston blow-by, all resulting in improved vehicle fuel economy of 2-4% for a gasoline powered vehicle; (ii) reduces hydrocarbon emissions; and (iii) reduces engine vibration by at least 20% at wide-open throttle conditions at moderate speeds (i.e., 1000-3000 rpm).